1. Field of the Invention
The present invention relates to a method of producing a glass particles deposit by depositing glass particles on a starting rod in a radial direction of the starting rod while relatively moving the starting rod and burners for synthesizing glass particles. Specially, the present invention relates to a method of producing glass particles deposit such that the glass particles deposit with a smaller noneffective portion can be obtained; an amount of glass particles to be deposited is finely adjustable; and deposition efficiency of glass particles is high.
2. Description of the Related Art
There is a method of producing a large-sized optical fiber preform at high speed (a multi-layers soot deposition), comprising the steps of, as shown in FIG. 5, disposing a plurality of glass particle synthesizing burners 7 at predetermined intervals opposite to a starting rod 1 in a vessel 4, and depositing glass particles (soot) in layers on the surface of the starting rod 1 while relatively reciprocally moving the rotating starting rod 1 and a row of burners 7, so that a glass particles deposit (soot body) 6 is obtained. FIG. 5 shows an example of the starting rod 1 that is vertically reciprocally moved.
In the method of depositing glass particles (soot deposition method), the relative movement distance (may also be called the traverse distance) between the starting rod and the burners may be set substantially equal to the distance between adjacent burners (hereinafter called the burner interval) in order to obtain the high deposition efficiency of the glass particles. In this case, there is a problem that the fluctuation of the outer diameter of the glass particles deposit (soot body) occurs in a longitudinal direction of the soot body. The outer diameter fluctuation of the soot body is caused by the fact that there arises a moment when the relative movement (may be also be called the traverse) between the starting rod and the burners always stops at a turn-back position (may also be called the traverse end portion) of the relative movement. Namely, the substantial time for depositing the soot (hereinafter called the soot deposition time) at the turn-back position tends to become longer than that is required in a normal portion where the relative movement is made at a steady rate. Further, the degree of touching of the burner flame onto the soot body at the turn-back position is different from that of the normal portion. In case where the relative movement distance is constant, the turn back position always falls on the same position of the starting rod, so that the fluctuation of the outer diameter is promoted. Generally, more soot is deposited in a portion whose surface area is large. Therefore, once the outer diameter of a portion becomes larger than other portion, the growth of the outer diameter difference among them is accelerated.
In order to obtain an optical fiber preform of good quality, it is important to reduce the fluctuation of the outer diameter of the soot body as much as possible and various methods for the purpose have been proposed.
For example, there has been proposed a method of reducing the fluctuation of such an outer diameter by moving the turn-back position of the traverse in a predetermined direction for each turn, and after the turn-back position is moved up to a predetermined position, moving the turn-back position of the traverse in the opposite direction for each turn until the turn-back position of the traverse is moved back to the initial position of the traverse. This method allows the traverse end portion of the traverse to be dispersed over the whole soot body. Therefore, the substantial soot deposition time and the fluctuation of the degree of touching of the burner flames onto the soot body are averaged over the whole soot body. As a result, the deposition amount of the soot can be uniform in the longitudinal direction thereof (Japanese Patent Unexamined Publication No. Hei. 3-228845).
There has been proposed another method of reducing the fluctuation of an outer diameter with Japanese Patent Unexamined Publication No. Hei. 3-228845 as a base by measuring the fluctuation of the outer diameter of the whole soot body and conducting, by an auxiliary burner, the additional soot deposition in a portion wherein the deposition quantity of the soot body is small (Japanese Patent Unexamined Publication No. Hei. 10-158025). The fluctuation of the outer diameter is measured by using a CCD camera capable of monitoring the whole area of the soot body and a central information processing unit. The auxiliary soot deposition burner can independently traverse the whole area of the soot body.
Moreover, there exists still another method of reducing a temperature gradient in the longitudinal direction of the soot body at the time of soot deposition by supplying clean air to the whole soot body in a direction perpendicular to the longitudinal direction of the soot body in the case of conducting the soot deposition while moving the turn-back position of the traverse (Japanese Patent Unexamined Publication No. Hei. 4-260618).
FIG. 6 shows a situation in which the relative position between the starting rod and the burners varies with time in the above-described method, and in which the tapered portion can be minimized in this kind of method. In FIG. 6, there are shown portions of the outermost burner 2 and the second burner 3 in a row of burners. Numerical values on the right-hand side indicate the number of deposited layers of soot on the starting rod 1 during a series of reciprocal movements until the turn-back position is returned to the initial position. As the soot formed by the second burner and thereafter are formed in portions below 18 layers of FIG. 6, the number of layers constantly remains to be 20.
In the case of the method of moving the turn-back position of the traverse, however, as shown in FIG. 6, the soot deposited by the burners positioned at both ends of the snot body is tapered down (because the number of deposited layers decreases to both end portions of the soot body). As seen from FIG. 6, although the number of deposited layers decreases in only the portions wherein soot is deposited by the burners at both ends, the soot deposited by a burner adjacent to the burners at both ends also tends to easily flow outside along the tapered shape. Therefore, most of the soot portions deposited by the second burners positioned from both ends become tapered, which results in forming noneffective portions.
An object of the present invention is to provide a method of producing glass particles deposit such that the glass particles deposit having a smaller noneffective portion can be obtained; an amount of glass particles to be deposited is finely adjustable; and deposition efficiency of glass particles is high.
As means for solving the foregoing problems, the invention is constituted of the following shown in (1) to (9):
(1) A method of producing glass particles deposit comprises: disposing a plurality of glass particle synthesizing burners opposite to a rotating starting rod; relatively reciprocally moving the starting rod and the glass particle synthesizing burners in parallel to an axial direction of the starting rod; depositing glass particles synthesized by the burners on the surface of the starting rod, wherein the relative movement is stopped and restarted while it is made from one turn-back position up to the other turn-back position of the reciprocal movement. The relative movement may be stopped and restarted during one reciprocal movement.
(2) A method of producing glass particles deposit as described in (1), wherein the plurality of glass particle synthesizing burners are disposed uniformly at predetermined burner intervals.
(3) A method of producing glass particles deposit as described in (2), wherein the plurality of glass particle synthesizing burners are disposed in a row in parallel to the starting rod.
(4) A method of producing glass particles deposit as described in (2), wherein a moving distance in one direction of the reciprocal movement is set twice of the burner interval or shorter.
(5) A method of producing glass particles deposit as described in (4), wherein the moving distance in the one direction of the reciprocal movement is set substantially equal to or substantially twice of the burner interval.
(6) A method of producing glass particles deposit as described in (1)-(5), wherein a distance between the adjacent stop points including turn-back points of the relative movement is within the range of 5 to 40 mm.
(7) A method of producing glass particles deposit as described in (1)-(6), wherein the step of depositing glass particles is terminated at a point of time when the burners arrive at the turn-back position of the reciprocal movement.
(8) A method of producing glass particles deposit as described in one of (1) to (7), wherein a stop time at stop point of the relative movement is set different from a stop time at the turn-back position of the reciprocal movement.
In the present invention, the condition brought about at the turn-back position in the reciprocal movement (stopping of the burner) is created positively in the middle of the movement. Therefore, it is possible to solve the problem of fluctuation of the outer diameter in the prior art. Further, it is possible to solve the problems of an increase in the noneffective portion formed in both end portions of the soot body, and a difficulty in precisely controlling the weight of the soot body and so forth.